The invention relates to a process for treating a powdered starting material based on natural calcium sulfate (gypsum) or synthesized calcium sulfate (sulfogypsum, phosfogypsum and other by-products of the same type) with a view to preparing a novel hydraulic binder that can be used as a cement, based on very high proportions of anhydrite III or xcex1 anhydrite (xcex1 in the ASTM classification).
The invention also relates to the product from said process, that can be used as a cement.
Gypsum has a variety of crystalline forms; on a molecular scale, it has a lamallar structure in which a layer of water alternates with two layers of CaSO4.
Gypsum has been used for thousands of years to produce plaster, one of the oldest construction materials, known since the 6th century BC.
Nowadays, other industries use gypsum, in particular:
for cement production (as a setting regulator);
for agricultural use;
in a variety of industries (chemicals, paper, etc.).
By far the most important use is its involvement in cement and plaster production, by dehydrating gypsum.
While in the cement industry, gypsum is incorporated into clinkering and is burned at a high temperature of the order of 1400xc2x0 C., when making plaster, the essential principle is to eliminate the water completely or partially from the gypsumxe2x80x94a complex operation that involves crystallization phenomena that are difficult to control.
Several types of calcium sulfate treatment processes have been proposed for preparing plaster. In particular, improved plaster (sometimes designated a plaster) can be prepared that, once hardened, has mechanical characteristics that are far superior to those of normal plaster. The phenomena that occur during such treatments are little understood and in general, the improvement in mechanical performance is attributed to the presence of anhydrite III or xcex1 anhydride in the products obtained, without accurately knowing the proportion of that type in the products, nor the conditions which can produce it in a stable and reproducible manner: it only exists in trace amounts.
Traditionally, improved plaster is produced from gypsum by burning under moist condition, in an autoclave, followed by a hot drying stage carried out in a stream of hot, dry air. Burning is carried out in a saturated steam atmosphere at a pressure of the order of 5 to 10 bars for a period of the order of 10 hours.
In order to try to overcome the drawbacks of that traditional process for producing improved plaster (a very expensive process with uncertain reproducibility), other processes have been proposed that attempt to reproduce the essential conditions of the traditional process (moist heat treatment followed by hot air drying) using different means and technologies (French patents FR-A-2 389 855, FR-A-2 445 940, FR-A-2 572 721, U.S. Pat. Nos. 2,269,580, 3,145,980).
The process of the invention was developed from the following observation: when calcium sulfate is treated normally to obtain an xe2x80x9cimproved plasterxe2x80x9d, the product obtained is in fact a mixture of anhydrous forms (xcex3 anhydrite) or hydrated forms (semihydrates, dihydrates, . . . ). The inventor""s studies have demonstrated that this state of affairs essentially derives from two factors. Firstly, burning produces xcex1 anhydrite and also other forms, and secondly, the product changes after burning, with partial transformation thereof, in particular re-hydration. The essential idea that has resulted in the process of the invention is to produce a stable final product containing a proportion by weight of xcex1 anhydrite that is much higher than that contained in known improved plaster; to this end, the structure of the compound obtained after burning is frozen by quenching. This considerably blocks the subsequent transformation of the xcex1 anhydrite formed by heat treatment.
PCT patent document PCT/FR96/00622 discloses that the quench operation is preferably carried out so that the material heated by burning is heated to a temperature of less than 100xc2x0 C. over a period in the range 6 to 12 minutes. It can be achieved using cold dry compressed air injected at a plurality of locations into the moving material, the flow rate of the air being adjusted to obtain a suitable cooling rate.
However, that quench is not sufficiently effective to produce a proportion of anhydrite III or xcex1 anhydrite that is really significant. That prior art patent application cannot produce a very high percentage of anhydrite III or xcex1 anhydrite, namely 90%, and thus cannot produce a hydraulic binder that can be used as a cement.
The process of the invention can produce such a product.
The aim of the invention is to refine the process to obtain a high proportion of stable, soluble anhydrite III or xcex1 anhydrite.
The process of the invention can transform more than 90% of pure calcium sulfate into anhydrite III or xcex1 anhydrite.
To this end, the present invention provides a process for synthesizing a hydraulic binder based on natural calcium sulfate (gypsum) or synthetic calcium sulfate (sulphogypsum, phosphogypsum, titanogypsum, etc), consisting in heating the calcium sulfate to form:
a hydraulic binder that can be used as a cement based on anhydrite III or xcex1 anhydrite, characterized in that it contains more than 70% of stable, soluble anhydrite III or xcex1 anhydrite and in that it consists in carrying out:
a heating or quenching step that brings the temperature of the treated gypsum from ambient temperature to a temperature in the range 220xc2x0 C. to 350xc2x0 C. depending on the characteristics of the treated gypsum;
a step for rapidly quenching the product obtained, reducing its temperature from 220xc2x0 C.-350xc2x0 C. to less than 80xc2x0 C. in less than two minutes, with the aim of stabilizing the xcex1 anhydrite by crystallographic blocking and fixing.
In one implementation of the process, the heating temperature is 300xc2x0 C.-310xc2x0 C.
In a preferred implementation, the temperature of the quench is in the range 40xc2x0 C. to 50xc2x0 C.
In a preferred implementation, the temperature is raised over a period of 10 to 40 minutes depending on the nature and grain size of the gypsum.
Prior to treatment, the treated gypsum comprises 0 to 20% of water, and its grain size is in the range 0 to 30 millimeters (mm).
In a preferred implementation, the treated gypsum comprises 5% to 15% of water and its grain size is in the range 0 to 10 mm.
A hydraulic binder that can be used as a cement based on anhydrite III or xcex1 anhydrite obtained by the above process is characterized in that it contains more than 70% of stable and soluble anhydrite III or xcex1 anhydrite.
A hydraulic binder that can be used as a cement based on anhydrite III or xcex1 anhydrite obtained by the above process is characterized in that it contains more than 90% of stable or soluble anhydrite III or xcex1 anhydrite.
A hydraulic binder that can be used as a cement based on anhydrite III or xcex1 anhydrite obtained by the above process is characterized in that the mechanical strength is:
22 MPa at 24 hours;
30 MPa at 8 days;
more than 40 MPa at 14 days.
The present invention also concerns a hydraulic binder that can be used as a cement obtained by carrying out the above process.
The essential inventive concept of the invention is thus to increase the proportion of xcex1 anhydrite in the product, the essential means employed being to limit the change in the product after burning by rapid cooling. To further increase this proportion of xcex1 anhydrite, the inventor also applied himself to optimizing the burning operation to obtain the largest possible quantity of this variety following burning.
When heated, gypsum produces a series of hydrated or anhydrous products.
At about 100xc2x0 C., xcex1 or xcex2 semi-hydrates are obtained (depending respectively on whether steam pressure or free air is used) as defined in the reaction: 
At about 300xc2x0 C., anhydrite III or a very soluble but highly unstable anhydrite is obtained which immediately re-hydrates to the semi-hydrate in contact with water vapor: 
At about 300xc2x0 C. for the xcex1 semi-hydrate and 350xc2x0 C. for the xcex2, anhydrite III (or axcex1 anhydrite) is transformed into stable anhydrite II (overburnt): 
Anhydrite III slowly re-hydrates in contact with liquid water.
At about 1230xc2x0 C., a new transformation reaction occurs: 
The anhydrite I CaSO4 only re-hydrates with difficulty.
Beyond 1250xc2x0 C., the anhydrite I decomposes: 
Current industrial applications for plaster use only:
xcex1 semi-hydrate;
xcex2 semi-hydrate; and
anhydrite II (insoluble or overburnt).
The anhydrite II or xcex1 anhydrite of the present invention could not be used because of its high instability.
The variation in the properties of plaster in buildings and its numerous weaknesses as regards strength, water resistance, adhesion to certain supports, etc, is also known.
The present invention concerns a hydraulic binder obtained by a specific and novel gypsum heat treatment comprising two essential phases. The first, dehydration, phase, produces a high percentage of anhydrite III (or xcex1 anhydrite) CaSO4. The second, rapid cooling, phase xe2x80x9cblocksxe2x80x9d the crystallography, rendering the anhydrite III (or xcex1 anhydrite) CaSO4 stable and making it capable of use
This rapid cooling, or quenching, in a dry atmosphere has never been carried out in the plaster industry.
Commercial plaster is obtained solely by dehydration and burning gypsum without any quenching, which latter constitutes the principal aspect of the invention.
The technical conditions for this novel hydraulic binder of the invention comprise:
1. A dehydration phase comprising raising the temperature of a low humidity gypsum (3% to 15% of water) to be treated; the temperature rise takes place over a period of 10 to 40 minutes, to a temperature of 220xc2x0 C. to 350xc2x0 C. depending on the nature of the gypsum, or more precisely between 300xc2x0 C. and 310xc2x0 C.
2. Rapid cooling or quenching in a dry atmosphere.
This rapid cooling, or quenching, has never been envisaged in the gypsum industry and stabilizes the highly soluble anhydrite III (or xcex1 anhydrite) CaSO4 anhydrite, blocking its crystallization by the thermal shock which must reduce the temperature from 220-350xc2x0 C. to less than 80xc2x0 C. in less than two minutes.
The high percentage of stable and soluble anhydrite III (or xcex1 anhydrite) CaSO4 (more than 70%, or even 90%) produces a remarkable hydraulic binder that can compete favorably with the majority of current binders.
This novel binder has the following characteristics:
fire resistance: inflammable, category Mo under French standard NF P 92-507;
excellent setting in seawater;
remarkable adhesion to all supports; and
setting at very low or high temperatures, etc.
The industrial applications for this binder are such that it is of interest to the cement, concrete and plaster industries, in rendering waste or industrial by-products inert, in the production of mixtures with materials that combine well with calcium sulfate, in building shelters in developing countries, etc.
This hydraulic binder can be produced by employing known techniques: low temperature burning (220xc2x0 C. to 350xc2x0 C.) and cooling. It can he carried out in very simple plant.
In addition to its technical qualities, this hydraulic binder is of:
economic importance;
energy saving importance;
ecological importance: non-polluting.
After complete dehydration, the percentage of anhydrite III (xcex1 anhydrite) CaSO4 is over 50%, or even 70% to 80%, while the rapid cooling causes crystallization of the form III (or xcex1 anhydrite) CaSO4 rendering it stable and soluble and enabling it to be used industrially.
The following implementation conditions appear to provide the best results. The amount of moisture in the starting material is first checked and then this amount is adjusted, if necessary, to a value substantially in the range 12% to 20%; burning is then carried out, bringing the powdered material to temperature by heating it under conditions sufficient to raise the temperature of the gas above the bed of material to a value in the range 350xc2x0 C. to 500xc2x0 C., and to bring the mean temperature of the material core to a value of over 220xc2x0 C. and below 350xc2x0 C. Heating can, for example, be carried out using infrared radiant heaters located above the bed of material, the power of said heaters being adjusted in correlation with the length of exposure of the material.
One possible explanation for the best results obtained with these operating conditions is as follows.
The temperature of 220-350xc2x0 C. in the core of the bed of material is ideal for producing xcex1 anhydrite and no other forms. The extracted water escapes from the bed of material into an atmosphere that is hotter, wherein the temperature is higher than its critical point (365xc2x0 C.). Thus, it rapidly reaches a supercritical state, preventing or limiting rehydration and the material surface changes such that when burning is finished, the proportion of xcex1 anhydrite is very high (it is not possible to give precise proportions as samples taken prior to cooling change immediately).
The exothermic transformation of xcex1 anhydrite to bassanite is very fast and is blocked by cooling that stabilizes the xcex1 anhydrite. Further, it appears that cooling completely blocks the change of the xcex1 anhydrite to bassanite plaster that is only found in trace amounts in the final product (in contrast to known binders that contain a large proportion of this form).
The product obtained when the binder of the invention sets (without filler) underwent flame resistance tests in accordance with French standard NTF P 92-507 (0.30 meter (m)/0.40 m samples subjected to radiation from a constant heat source) Determination of the four indices defined in the standard (flame index, development index, maximum flame length index, combustibility index) enabled the product to be placed in category MO, the highest of the six categories defined in the standard.
Further, strength tests carried out in accordance with the standard produced the following results:
compressive strength: 40 MPa; and
bending strength: 10 MPa.
In addition, qualitative tests carried out on immersed samples showed that the strength of the product remained good in such a situation.
The study below, describing calcium sulfate dehydration, enabled the process of the invention to be perfected to obtain a true hydraulic binder that can be used as a cement.